Experiencing The Cutting Edge Of Dredging Technology

Exhibit on cutting forces at the Damen Dredging Experience
Exhibit on cutting forces at the Damen Dredging Experience

Ever tried to eat peas with a knife? Didn’t fare well, right? Sorry, this post will not help you. It is just more entertaining doing so. And you might remember this week’s lesson.

After all the other posts, I would like to continue our tour along the exhibits of the Damen Dredging Experience. We’ve seen at “the Bank” that by gravity and hydraulic action the sand can start moving towards the suction mouth. Another well-known mechanism for collecting the sand is by cutting. This mechanism is primarily used in the cutter heads of CSD’s of course, but also in the trailing drag heads of TSHD’s.

Be aware, that the gravel sample in the exhibit is an artificial sediment, specifically designed to be porous and demonstrate the “rolling peas effect”. Naturally occurring gravel sediments have a wide range Particle Size Distribution and will have virtually no gaps between the grains. The smaller grains will lock the bigger grains in place and it will be more difficult to move them.

In this exhibit you will see two different types of sediment. Wet fine grained sand and very coarse gravel in a jar with a handle. When you rotate the blade in the gravel, you will notice a rather high cutting force, that remains relatively constant. The water can easily move through the pores. Rotating the blade in the jar with sand, is very hard at first. But as soon, as there is a chunk of sand dislodged, the cutting force is decreased dramatically. The decrease can be explained by the fact, that once water has entered the shear plane, it cannot dissipate back in the pores and will lubricate the chunk moving over the bed. This is a phenomenon, sometimes encountered with ploughs.

Under pressure in densely packed sediments
Under pressure in densely packed sediments

The theory of cutting sand is largely explored by dr. S.A. Miedema1. He wrote an extensive book2 on this subject and anyone interested in the details is encouraged to read it. Again, the basis for cutting sand is the dilatancy of sand, just like in the previous exhibit. The grains are moved and the water has to be forced in the pores. As the blade moves at a certain speed, the sand exerts a horizontal cutting force on the blade. Force, times speed is power. At this speed and cutting height you have a certain production. Power divided by production yields a Specific Cutting Energy3, which is a parameter for how much effort it costs to cut 1m³. The SCE is largely governed by the undrained shear strength and the angle of internal friction and is different for every type of sediment. They are measured with a Cone Penetration Test4,5. In order to estimate the production of the dredge, we really need to know these parameters. If they are not available, maybe you can receive the results from a Standard Penetration Test6,7.

Basic explanation of the theory of sand cutting
Basic explanation of the theory of sand cutting

From the equations, you can derive that for a hard material, the SCE can be quite high. Consequently, with a known installed cutter power, the production Q will result quite low. From this perspective, there is no upper limit in the hardness of the soil, anything can be cut. It is just, that the resulting production might be too low for a viable business case. In this respect, it is always difficult to say the maximum hardness of the soil the CSD can cut. Usually, the increased vibrations and unsteady process will limit the productivity in such circumstances.

Example of a cutter production for a CSD
Example of a cutter production for a CSD

This provokes a nice practical experiment for you at home or when you have you have to entertain guests at dinner: have a nice recipe with big peas and fine grained rice8. Serve the peas and rice separately and notice the variation of effort to stir the ingredients separately. Then, mix a portion together and notice the increase in cutting force. For enhanced realism, add some sauce. Exclaim your amazement to your perplexed table partners and explain that you are not playing with your food, but are on a study assignment for your work. Bon Appetit!

Ingredients for a pea and rice recipe
Ingredients for a pea and rice recipe

References

  1. dr. S.A. Miedema, TU Delft
  2. The Delft Sand, Clay & Rock Cutting Model, Dredging Engineering
  3. CEDA Webinar Specific Cutting Energy, CEDA
  4. Cone penetration test, Wikipedia
  5. Painted Hills, how to unveil the sediment layers below the surface, Discover Dredging
  6. Standard penetration test, Wikipedia
  7. Lessons in Camping: Basic Soil Investigation, Discover Dredging
  8. Nice rice-a-pea, Albert Heijn

See also

Loose Sand, How Hard Can it Be?

Breaching exhibit at the Damen Dredging Experience

Did you ever tried to build a sand castle? Probably yes. Felt frustrated it always collapsed unexpectedly? At least I did when I was a child. But it took me an academic study to know why. Lucky you. You just have to read this blog post and experience a moment of enlightenment. So, this is good moment to stand up and get some coffee. You will enjoy reading it more and remember my lecture every moment you take a sip.

The second exhibit in the Damen Dredging Experience is an installation, which we call: ‘the Bank’. Usually there is some mechanical or hydraulic action, that will cause the sediment, to become unstable. In this exhibit, we can turn the little wheel at the lower right corner. The first thing you will notice is that at the higher end of the soil surface, the grains will slightly move and start to tumble down along the slope. Where the activation of the particles start at the slope is called the active bank.

Breaching the bank and density flow

The effect we would like to demonstrate, is that different soil types, do have different behaviour in this process. There are three different soil types, from course to fine, from the front to the back. The finer material at the back seams to stay the longest at rest. This is due to a phenomenon that we call ‘dilatancy’1. If a stack of grains is sheared, they have to hobble over the tops of the layer below.

Under pressure due to dilatency on a shear plane

When the grains do hopscotch over each other, they require more space to do so. Effectively the pores increase in volume and the total sediment expands. The extra space cannot be accommodated for by expanding water, it has to be replenished. The extra water has to come from the outside. But the grains themselves are in the way and form resistance to the incoming water. The resistance causes a differential pressure under the ambient pressure, commonly known as ‘vacuum’. And grains under vacuum tend to cling together and form chunks. This happens mostly, when the pores are small, or when the grains are small. Exactly what you can see in the exhibit.

Once the sediment is loosened from the active bank, it rolls down the slope, it behaves like a dense fluid, driven by gravity2. When the slope becomes less, or the running fluid encounters resistance, the sediment will settle again at the so called ‘passive bank’.

Outflow of density current and sedimentation

Here the reverse process happens, the water has more trouble getting out of the suspended flow and run longer. The passive slope will be flatter at finer grains than in more coarse material.

Both processes can be identified in e.g. the DOP3. It is usually suspended on a wire and lowered onto the seabed. Powerful jets excavate a small pit where the suction head takes up the suspended material. The walls of the pit become unstable as an active bank. The loose material flows into the pit. This turns into a continuous process and the active bank, runs away from the suction pit.

Breaching and density flow in a DOP process

Now, it is immediately evident, why DOP pumps have this characteristic suction pipe. It fits snugly in the pit and has the least resistance for the incoming density flow. Another benefit of the suction tube, is that if the bank collapses on the DOP, the suction pipe can be extracted without too much trouble. Extracting a pump from under a collapsed bank imposes the same trouble as creating a passive bank: suction due to dilatancy.

So, your sand castle collapses when water enters the pores. A demonstration of grains becoming as strong as a concrete block by under pressure is a well-known household phenomenon: vacuum packed coffee. Now, you will think of this, whenever you open a new pack of coffee.

Vacuum packed coffee is stable due to under pressure in the pores

References

  1. Dilatancy, Wikipedia
  2. Density current, Wikipedia
  3. DOP pumps, Damen

See also

A Sample of Soil Samples

Soil sample exhibit at the Damen Dredging Experience

‘Welcome Sir, very nice you would like to buy our dredge. What do you want to dredge with it?’
‘Sand.’
‘Ah Sir, very well. Our dredges are very capable of dredging sand. What kind of sand are you going to dredge?’
‘Grey sand!…’

Sounds familiar? Well to me it does. Sometimes, even our most esteemed customers lack a basic knowledge of their primary process. Often, I’ve been called into a meeting with the customer, to explain about sand and its physical properties. Nowadays, we can show them an exhibit in the Damen Dredging Experience1 to discuss their particular case. Eventually, we can tease out the information we need, to inform the customer the estimated production of their dredge. Educated customers do know about their operation and can make the estimation themselves.

There are several parameters that influence the performance of the dredge. Particle sizes, grain forms, densities, mineral types, cohesion and many more. And we are very happy when the customer already has his own soil report of his particular operation. This would comprise bore logs, particle size distribution and cone penetration tests. If this is not possible, a sample of the concerned soil will do. We do have our own small soil mechanics lab, in which we can measure the required properties.

Elementary soil mechanics laboratory at our company

The most useful property is the particle size distribution or PSD. This can be done in a sieve tower. (red circle) The soil sample is placed in the top sieve and the sieves are vibrated to separate the various fractions. The contents of the sieves are weighed and plotted in a logarithmic graph. This resulting PSD can then be used in production estimations.

Procedure to establish a particle size distribution

For a good measurement, we need a sample the size of a 1.5L coke bottle. About two thirds of soil and one third of water. We need the water to capture the fines in the sample. Do not drain it! And the bottle is a good container for transport and widely available. Stash it in your check-in luggage. Otherwise, you will run into trouble with the airport security about carrying liquids in your hand luggage.

Examples of sample containers encountered for estimation

All these variations in soil properties have a major influence on the performance of the dredge. So, this is why we would like to know exactly what the customer is going to do with our dredge. Otherwise, we might end up in an analogous situation when somebody wants to buy a truck. You can explain all about the installed power and the cargo capacity and then you get this question:
‘Sir, can you tell me how much paper I can transport with this truck?’
‘Ah well, we are not in the transport business ourselves, but as you are a good customer, we can make an estimate for your convenience. What kind of paper? Kite paper? Cardstock? In blocks, boxes or roles?’
‘Oh, I don’t know. Just paper’.

Model dump truck from a customer2, typically used at dredging projects

References

  1. Kommer Damen opening the ‘Damen Dredging Experience’, DredgingToday
  2. 2. 35 ton dumper, Martens en Van Oord

See also